This invention relates to a new and improved fuel feed system for diesel engines, and more specifically, to a fuel feed system which supplies fuel to the engine at controlled temperatures to improve engine performance. Water, LPG and alcohol injection may be utilized to reduce fuel consumption, lower NO.sub.x emissions and reduce engine deposits.
Various problems are encountered when starting or operating large diesel truck engines in cold weather, a principle one being the use of cold fuel. Usually, the fuel must be pre-heated and this is both time consuming and expensive.
These problems have become somewhat more aggravated in recent years due to the increased use of higher boiling point wax fractions in the fuel. These wax fractions tend to crystallize out in engine parts and fuel lines more readily in cold weather and make starting and operating even more difficult.
In addition to starting problems, diesel engines are designed for optimum performance using fuel within a certain temperature range. For example, typical Cummins engines operate best with fuel in the temperature range of about 94.degree. F. to 104.degree. F. and it is most important that the fuel temperature be controlled to within this range at all times. Hence, in the summer, the fuel should be cooled down to about 94.degree. F., while in the winter, the fuel should be heated in excess of about 94.degree. F. Accordingly, if the engine fuel runs too cold or too hot, performance will be impaired because the feed rates for fuel and air are optimized when operating within the above temperature range. Thus, during cold starting, it is obviously important to employ a warm or hot fuel depending on ambient temperature. On the other hand, after running temperatures are attained, it may be preferable to supply the fuel at cooler temperatures, particularly if the engine is running in an excessively hot ambient temperature.
Normally, part of the cooling in a diesel engine is accomplished via fuel returned to the main fuel tank; the returned fuel represents about three times the fuel actually burned. However, when the ambient temperature exceeds about 100.degree. F., the main fuel tank temperature may approach about 180.degree. F. which is nearly at the engine running temperature.
It has been found that a fuel density decrease due to a temperature increase will cause a 1% decrease in horsepower for each 10.degree. F. rise in fuel temperature above about 94.degree. F. Hence, if the engine temperature is 180.degree. F., and the fuel tank temperature is, say, 160.degree. F., the horsepower loss is approximately 7.5% depending on the fuel density curve. Conversely, if the ambient temperature is too cold, the return fuel will not heat the fuel tank to 94.degree. F.; this will cause the fuel density to increase and exceed the air supply to the engine intake. Consequently, the fuel supply will be too rich, causing poor combustion characteristics and carbon deposits to form. Overall, fuel atomization of the fuel injection is optimized when the fuel at the inlet pump is about 94.degree. F.-104.degree. F. and the engine temperature is a constant 180.degree. F.
One solution for effectively controlling fuel temperature to accommodate rapidly changing engine conditions caused by ambient temperature, engine load and traffic conditions would be to heat or cool the fuel tank. However, this would require excessive power in the case of heating, and is impractical in the case of cooling.
While earlier engines were low powered with a large number of gear shiftings to accommodate for various speeds, loads, grades, etc., the newer diesel engines have a greater horsepower at lower rpm and fewer gear shiftings. This in turn enables faster trip times and hence a more economical operation. However, these newer engines are more sensitive to fuel temperature control in order to achieve their maximum design performance, and the increased cost of fuel has reduced their economic gains.
The desire for water injection or supplemental fuel injection into the engine air intake manifold has known desireable effects such as increased power, but adequate delivery systems for fuel and water have proven unsuitable, and a satisfactory temperature control system for fuel has not been developed.